This invention relates to a single mode optical fiber, and more particularly a single mode optical fiber in which a core is surrounded by a cladding.
Various efforts have been made to improve the single mode optical fiber to meet requirements raised by the development of optical communication and in recent years improved single mode optical fibers have been developed which in addition to ordinary characteristics of light weight, noninductive and no cross-talking have a lower loss than a transmission line utilizing metal conductors such as a millimeter wave guide and can transmit data over a wider frequency band. A long distance ultra wide frequency band optical communication system operating at a frequency of 1 GHz and without using any repeating station, is now being planned by using optical fibers.
It has been theoretically determined that it is necessary to reduce the total dispersion to be less than .+-.1 ps/km/nm and to reduce the transmission loss to be less than 0.5 dB/km in order to realize such long distance ultra wide band transmission. Considering the transmission characteristics of present day optical fibers from this standpoint of view, the ultra wide band long distance transmission involves certain problems for the following reason.
In an optical fiber, light entering into the core transmits therethrough by repeatedly being reflected at the interface between the core and the cladding, and deterioration of the transmission characteristics caused by the dispersion of light can be analyzed as follows: (1) multimode dispersion.
This is caused by the fact that the propagation constant representing the propagation state of the light varies nonlinearly with respect to the angular frequency of the light and that as the angular frequency increases, higher order modes appear in the light propagating mode so that group velocities vary for different modes. (2) material dispersion (.delta..sub.M).
This is caused by the fact that the refractive index of glass constituting the optical fiber varies nonlinearly and that the material dispersion .delta..sub.M of a single mode optical fiber is shown by the following equation ##EQU1## where n.sub.1 represents the refractive index of the core, .lambda. the wavelength of light and C the light velocity in vacuum. (3) waveguide dispersion .delta..sub.W.
The waveguide dispersion is determined by the relationship between the propagation constant .beta. and the angular frequency of light .omega. and is expressed by the following equation in the case of the single mode optical fiber ##EQU2##
In the case of a multimode optical fiber it is necessary to take into consideration the three dispersions described above, but the invention is directed to a single mode optical fiber for the purpose of decreasing signal distortion by minimizing as far as possible the dispersions. Hence, it is not necessary to consider the multimode dispersion but only the material dispersion and the waveguide dispersion must be considered. Accordingly, in the case of the single mode optical fiber a sum (.delta..sub.M +.delta..sub.W) of the material dispersion .delta..sub.M and the waveguide dispersion .delta..sub.W constitutes the total dispersion .delta..sub.T and the frequency bandwidth f in which signals can be transmitted without distortion is expressed by the following equation. EQU f=(0.187)/(.delta..sub.T) (3)
In a typical prior art optical fiber the diameter of the core 2a=9.0 microns, the refractive index of the core n.sub.1 =1.46319, and the difference in the refractive indices of the core and cladding ##EQU3## Examples of such single mode fiber are described in a K. Daikoku et al. paper entitled "Direct Measurement of Wavelength Dispersion in Optical Fibres--Difference Method", Electronics Letters, 1978, Vol. 14, No. 5, pages 149-151.
With such prior art optical fibers, when the wavelength of light varies in a range of 0.9-1.6 microns, for example, the material dispersion varies in a range of from +66 ps/km/nm to -25 ps/km/nm and the difference in the refractive index and the core diameter has values described in the preceeding paragraph, the waveguide dispersion .delta..sub.W varies in a range of from 4 ps/km/nm to 13 ps/km/nm. For this reason, the total dispersion .delta..sub.T varies in the positive and negative directions about .lambda.=1.43 microns. Accordingly, when the optical fiber is operated at a wavelength .lambda.=1.43 microns at which the total dispersion .delta..sub.T becomes zero the distortion of the signal becomes minimum so that transmission over a long distance at a wide bandwidth becomes possible. However, if the wavelength differs from 1.43 microns even slightly, the total dispersion .delta..sub.T would increase greatly with the result that the frequency bandwidth f shown in equation (3) rapidly decreases.
Accordingly, with the prior art optical fiber, even when the total dispersion .delta..sub.T is made to be equal to .+-.1 ps/km/nm suitable for long distance transmission, the usable wavelength width is at most from 1.41-1.43 microns. For this reason, for long distance wide bandwidth transmission, even when the wavelength is divided or multiplexed for the purpose of improving the transmission efficiency, it has been impossible to operate the optical fiber in the wavelength range described above.